1. General

At this point in the quarter, it would be very advisable to review your infrared spectroscopy and NMR spectroscopy notes from lecture. You are expected to have mastered these topics by the time the final exam comes around (in about four weeks). You will need to memorize the most important numbers and trends for the exam because there will be no tables provided!

Note for the teaching assistant: the students should start the reflux first and then take the quiz while the reflux stabilizes.

2. Experiment

Safety notes:

1. Concentrated sulfuric acid is very corrosive. Wear nitrile gloves when you transfer it! Avoid spills on your clothes because it will ruin them! Make sure to keep the supply bottle closed to avoid the absorption of water from the air because it is hygroscopic.

2. Diethyl ether, the unknown alcohols and some of the esters are flammable. Keep them away from any ignition sources i.e., hot plates, etc.

3. While it is highly advisable to acquire the refractive index of the unknown alcohol, the melting  point of the carboxylic acid is often not useful because the acids are of lower quality (hence lower mp and the yellow and beige colors).

Why is it very important that the glassware is dry?

Why is the alcohol used in a five-fold molar excess compared to the carboxylic acid?

The drying tube (= L-shaped thing in your desk) is packed with granular anyhdrous CaCl₂ and placed onto the air condenser. The proper packing of the drying tube is a loose cotton plug first, then about 4-5 cm of CaCl₂ and finally another loose cotton plug. The cotton plug are supposed to keep the calcium chloride in place but should not provide a tight seal.

After the reaction, the aqueous layer and the organic layer are separated. The aqueous layer is then extracted with two portions of diethyl ether. The organic layers are combined and then carefully extracted with sodium bicarbonate in the conical vial or centrifuge tube. In the beginning, the two layers are mixed with a spatula or a small glass rod (especially the first time). At this point, it is very important to vent the container frequently. Why?

If there is no layer separation is observed upon the addition of water, some solid sodium chloride can be added. This will increase the polarity of the aqueous layer, which in many cases causes a phase separation. If this does not occur, the mixture is extracted with diethyl ether and processed as above.

The amount of the drying agent used should be minimized in order to minimize the loss of product that also adsorbs onto the drying agent.

Before the vacuum distillation can be carried out, the diethyl ether has to be boiled off using the rotary evaporator.

In order to obtain a good vacuum, the ground glass joints of the glassware have to lubricated lightly. The vacuum tubing (check for cracks here) has to fit snuggly over the empty drying tube. If it was too hard to push the vacuum tubing over the vacuum tube, the drying tube should be lubricated lightly as well. The better the setup is sealed, the easier and faster the vacuum distillation is going to be.

During the distillation, the setup is first evacuated without heating to remove the diethyl ether that was not removed prior to this step. The setup is heated gently in vacuo to remove the leftover alcohol. The Hickman head and the air condenser is not collected at this point. The alcohol is sucked into the vacuum line. If the solution stops boiling, the temperature is increased again to distill the ester into the Hickman head.

The product has to be characterized using refractive index and infrared spectrum by the student. The remaining sample is submitted to the TA in a properly labeled vial (your name, the combination (i.e. Z9, your TA and section) so that the TA can prepare the NMR sample for you .Make sure to sign in your sample as well. The printouts can be picked up in the instructor's office.


3. Things to think about

a. Why is an excess of acetic acid  and not an excess of isopentyl alcohol used in the synthesis of isopentyl acetate?
b. Why does the esterification not go to 100 % completion? How can the yield of the ester be improved using the same reagents?
c. Why are esters very important compounds in industry?
d. Why do you have to wrap a wet paper towel around the air condenser during the distillation?
e. Why do you have to heat the mixture to a reflux?
f. Why is the vacuum distillation performed in the end of the purification?
g. Why is sodium bicarbonate used for the neutralization and not sodium hydroxide?
h. Why is it important to keep the calcium chloride out of the reaction mixture?